Colored optical elements



United States Patent Ofl ice y, 3,123,485 ?atented Mar. 3, 1964 This invention relates to light-transmitting, plate-like crystals of colorless substances, which assume color because of light interference phenomena.

The crystals of the present invention may be utilized in the production of interference filters, in nacre-producing compositions such as nacreous pigments, or for decorative effects in which multiple colors are obtained by use of a single color-producing ingredient.

Light interference produces color in films of the appropriate thickness. When the thickness of the film varies sufficiently, a Whole rainbow of colors is seen. When the film has constant thickness, there is no spectrum, but the color depends on the thickness, the angle of observation, and Whether the film is observed by transmitted or by retlected light.

Interference films have been prepared by a number of techniques. in one, the film is deposited by evaporation and condensation of a suitable substance on a smooth surface. In another, a surface is coated with a dilute solution of a film-forming substance, the thickness of the Wet coating and the concentration of the film former being such as to leave a dry film of the desired thickness.

According to the zresent invention, the interference of light i brought about by means of crystals which are transparent plates of the desired thickness. These crystals, incorporated in a suitable light transmitting medium, behave like interference films, even though they consist of discrete units instead of a single, continuous film.

Plate-like crystals are ordinarily used in pearlescent (or nacreous) pigments, Where reflections of light from many layers of transparent rystals, which are oriented parallel to one another, combine to produce a luster resembling that of the pearl. The conventional pearlescent pigments give an effect of whiteness because the crystals are either of a thickness below that which produces interference effects, or else are so heterogeneous in thickness that no single interference color is formed.

The crystals of this invention are plates having broad "aces which are smooth and substantially parallel to one another, all the crystals having essentially the same thickncss. As will hereinafter become apparent, a two-color, or partial iridescent elfect be achieved when color arises from interference phenomena in such crystal platelets.

Because there has been some confusion in the art in the use of the words iridescent, pearlescent, and nacre- :us, the term pearlescent and nacreous as herein defined shall refer to the lustrous appearance of the pearl or mother-of-pearl shell, without reference to the play of colors sometimes seen in these natural objects. The conventional pearlescent pigments produce this luster without any color play, the finished object being essentially White unless dyes, colored pigments or other coloring agents are added. Although the ordinary pearlescent or nacreous et tect has sometimes been referred to as iridescent" this effect does not by itself, produce color. iridescence as herein contemplated describes a play of colors such as that seen in the soap bubble.

The interference phenomena utilized in the formation of color in this invention depend on the fact that a thin crystal platelet is essentially a thin film. When White light is reflected by a thin film, the rays from the two surfaces may interact, resulting in the reinforcement or the destruction of light of certain Wave lengths. A change in the composition of the light, brought about by such reinforcement or destruction, produces color.

The know equations which govern the color effects depend on the indices of refraction of the thin film or platelet and the surrounding medium. Thus, destructive interference occurs if the reflections from the two opposed surfaces of the platelet are completely out of phase. This is the case for wavelength A (for light perpendicularly incident on the film) When where d is the thickness of the film Whose index of refraction is N and n is the order of the reflection. This familiar equation governs destructive interference in the reflection from a thin film; the index of refraction of the surrounding medium does not appear in the expression in this special case of normal incidence.

if the incident light is monochromatic and of wavelength A there is no reflection at all under these conditions. If on the other hand, the crystal plate is illuminated by White light, all wavelengths except A will appear in the reflection.

The above equation and the other known equations which describe interference eifects depend on the index of refraction of the thin film or crystal platelet. Thus with a plate of inaex 2.0, rein orcement effects are obtained for thicknesses of approximately 50 to millimicrons (ma), While destructive interference effects are observed with thicknesses of from approximately 95 to 180 my.

Color effects of maximum intensity are obtained in the range of about to 206 m although good color appears with thicknesses as high as 1090 mg. The reinforcement colors of crystals less than 90 Eli thick are in part diluted by scattering of blue light by these very thin crystals; hence these reinforcement colors are not in the preferred range.

inasmuch as the plate-like crystals herein contemplated may be of different compositions and differ as to their index of refraction the conditions for producing the desired color effects can best be defined in terms of the multiplication product of index of refraction and crvstal thickness. Thus, where thickness is expressed in millimicrons, color effects of maximum intensity are obtained at product values of at least 200, good color appearing up to 2000. The preferred range is 200 to 400.

In accordance with this phenomenon, if, for example, destructive interference of green takes place on reflection by the crystal platelet, the color of the reflected light consists of the composite of all the residual wave lengths, and is red. The light transmitted by the plate, on the other hand, is green, consisting in large part of the Wave length which was destroyed by reflection. Thus, the reflected and transmitted colors are complementary to one another, making possible two-color or iridescent etiects.

Crystals which are suitable for the production of interference colors must be capable of growth in the form of plates with smooth surfaces and uniform thickness. Among the suitable crystals are lead hydrogen phosphate, lead hydrogen arsenate, basic lead carbonate, bismuth oxychloride, mercurous formats, zinc phosphate, cytosine, and DL-tryptophane. All of these substances can be crystallized as plates which become colored on attaining the proper thickness, in accordance with the present invention. For practical purposes, the plate-like crystal should have an index of refraction which differs as much as possible from the substance in which it will be embedded in use, since the intensity or" the reflected light is a function of the difference in refractive index between the film or crystal and the surrounding medium. Generally, the plates should have an index of refraction of at least 1.70, and the multiplication product of index of refraction and thickness should be at least 200 to 400, where thickness is expressed in millimicrons.

The common light-transmitting plastics and film-forming materials are the most convenient embedding or supporting media for the crystals, those which are clear and colorless being preferred for most applications. These materials, which are Well known as supporting media or vehicles for nacreous pigments, usually have refractive indices in the region of 1.4 to 1.65, and would include such film-forming media as cellulose nitrate, cellulose acetate, cellulose acetate butyrate, ethyl cellulose, polyvinyl chloride and its copolymers, polystyrene, and alkyd and acrylic resins. Typical plastics would include polyesters, epoxies, acrylics such as polymethyl methacrylate, polyethylene, casein, and phenol-melamine and urea formaldehydes. It will be apparent to those skilled in the art that many of the above-listed compounds are suitable for use either as film-formers or in making plastic articles.

The clystal plates referred to which differ most from this refractive index range, and which are therefore the most effective include basic lead carbonate (indices 2.09, 1.94), lead hydrogen phosphate (approximately 1.84), and lead hydrogen arsenate (1.97, 1.90). The index of refraction of the crystal enters not only in the intensity of the reflected light, and therefore of the color effect, but also the thickness required to produce a particular color as is evident from the above equation.

It has been mentioned that all the crystals must be of essentially the same thickness in order to produce color effects of the desired intensity. As a practical criterion it may be considered that a minimum of about 80 percent of the total crystal plate area should not differ in thickness by more than percent of the average crystal thickness. In a satisfactory red-reflecting basic lead carbonate preparation, for example, as described in Example IV below, approximately 80 percent or more of the total plate area of the crystals which have an average thickness of 124 m should fall in the range of about 112 to about 136 m Crystal thickness of at least this degree of uniformity insures color of sufficient intensity for practical utilization.

Plate-like crystals of the desired dimensions are produced in accordance with this invention by either a digestion procedure in which the crystals are permitted to thicken until the desired plate thickness is attained or by direct growth to the desired thickness. The digestion procedure is particularly well suited to the production of crystals of a high degree of thickness uniformity, as illustrated in the following examples.

Example I Crystals of the lead hydrogen phosphate, PbHPO are precipitated by rapidly adding 50 pounds of a percent aqueous Pb(C H O solution to 200 pounds of a well agitated solution consisting of 10 percent H PO in water. The reaction is carried out at C. After stirring for five minutes, 3.5 pounds of concentrated HNO (70 percent) are added, and the suspension is heated with continuous but gentle stirring to 80 C. After ten hours at this temperature, the crystal suspension is seen to be golden in color by reflected light, corresponding to the destructive interference of the blue reflection of about 450 m wavelength. This corresponds to a crystal thickness of 122 m The average crystal length is approximately microns The resulting gold-reflecting (blue transmitting) crystals are centrifuged and washed by centrifugation with water first and then with isopropanol. The color effects of the resulting isopropanol suspension may be demonstrated by incorporating 0.50 gram of a 50 percent crystal suspension in 100 grams of a syrup made by the partial polymerization of methyl methacrylate monomer. After the addition of catalyst (such as 1.0 gram of 25 percent acetyl peroxide lead carbonate crystals.

in dimethyl phthalate), the crystal-syrup suspension is poured into a casting cell consisting of two glass plates held apart by a one-eighth inch gasket consisting of fiexible tubing. The cell is immersed in a Water bath at 50 C. for six hours, and on being opened, yields a polymethyl methacrylate sheet which is gold by reflected light and blue by transmitted light.

Example II The procedure of Example I is followed except that the digestion is carried on for eleven hours. The stirring crystals assume a red color, produced by the destructive interference of the green reflection at 525 mg. The crystal thickness is 143 mg, the average length approximately 40/!" Example III The procedure of Example I is followed except that the digestion is carried on for thirteen hours. The crystals now appear green in color, because of the destructive interference of the reflection at 670' mg. The corresponding plate thickness is approximately 182 mg. The average crystal length is approximately 5071..

Example IV The following procedure for the precipitation of basic lead carbonate crystals is based on that described in U.S. Patent No. 2,950,981 owned by applicants assignee. To 200 gallons of demineralized water at 25 C. (hardness equivalent to 2 ppm. as calcium carbonate) are added 2.7 pounds of propionic acid. To the stirring solution are added 12 pounds of lead oxide. After standing overnight, the basic lead propionate solution is filtered using filter aid; with the clear filtrate at 25 C. CO is introduced into the stirring solution to precipitate basic The pH value, which is originally at about 8.0, falls with the addition of CO the addition being terminated when the pH reaches 6.5. The CO addition period is approximately six hours. The crystal suspension is transferred to an autoclave, where it is heated to C., with continuous, gentle stirring. After eight hours, the previously White crystals appear red by reflected light, corresponding to a crystal thickness of 124 mg. The crystals are hexagonal in shape with an average diameter of about 30,41

Example V Lead hydrogen arsenate crystals are precipitated by a procedure similar to that of Gertzog United States Patent No. 2,311,533. To 100 pounds of 15 percent Pb(C H 0 solution at 95 C. are added, with stirring, 515 pounds of 13 percent H AsO solution, also at 95 C. After 10 minutes, 10.0 pounds of concentrated nitric acid (70 percent) are added. The suspension is kept at 95 C. with gentle stirring, for six hours, after which the initially white crystals appear green by reflected light, corresponding to a crystal thickness of about 170 mg; the crystals average about 40 in length.

In the above examples, various elevated temperatures have been used for the digestion of the crystalline suspension. It should be apparent that digestion can occur at room temperature to autoclave temperatures of C. or higher, depending on the amount of time devoted to this step and which can be readily made to vary from about 5 hours to one week.

It should be clear that in the above examples the crystals may be incorporated into various film-forming or plastic media of the type hereinbefore referred to, depending on the ultimate use desired.

The crystals of the preceding examples can be used in the economical preparation of Wide-band interference filters. For example, the crystals may be dehydrated by filtering and displacement of the water by a water-miscible solvent, such as isopropanol or monomethyl ether of ethylene glycol. They may then be incorporated into a suitable lacquer, and sprayed on a glass surface.

areas-s The crystals may also be used for decorative etlects, as in the simulation of mother-of-pearl. To ac ieve this efifect, a suspension of the crystals (red-reflecting, for example) in a polymerizable resin, e.g., a polyester, may be cast in the form of a sheet. The crystals are made to form a mottled pattern by drawing some object through the viscous suspension, thus causing the crystals to be oriented along the path of the object. When the finished sheet, after polymerization and curing, is held against a white background and observed in reflected light, it appears red where the crystals lie in position to give specular reflection, and green at other positions, Where the light reflected from the white back round is filtered through the plastic sheet. This iridescent pearl eiiect is normally not obtainable with dyes, which would cause the same color to be both reflected and transmitted. The plastic sheet with inherent red-green color play can be utilized in the production of polyester buttons, trays, screens, etc.

The same red-reflecting crystals provide unique color eiiects when coated on a curved surface. A suspension of the crystals in cellulose nitrate lacquer may be sprayed on a curved metal object which has been previously prepared with an ordinary white undercoat. The finished surface appears red at the highlights and green in all other positions, the colors shifting as the object is viewed from different angles. The coating also has a pearly luster.

The crystals can also be utilized by incorporation in plastics by the usual techniques, such as extrusion, injection molding and calendering.

Although this invention has been described with respect to its preferred embodiments it should be understood that many variations and modifications will now be obvious to those skilled in the art, and it is preferred, therefore, that the scope of this invention be limited not by the specific disclosure herein, but only by the appended claims.

What is claimed is:

1. Light-transmitting crystalline nacre-producing plate lets selected from the group consisting of basic lead carbonate, bismuth oxychloride, lead hydrogen arsenate, lead hydrogen phosphate and mercurous formate, having uniform thicknesses and an index of refraction of at least 1.70, the multiplication product of thickness expressed in millirnicrons, and index of refraction being in the range of about 260 to 2000, the uniformity of thickness of said platelets being such that at least about 86% of the total plate area does not differ in thickness by more than 110% of the average platelet thickness so that colors are produced by optical interference phenomena upon reflection and transmission of light by said platelets.

2. Light-transmitting crystalline nacre-producing platelets selected from the group consisting of basic lead carbonate, bismuth oxychloride, lead hydrogen arsenate, lead hydrogen phosphate and mercurous formate, having uniform thicknesses and an index of refraction of at least 1.70, the multiplication product of thickness expressed in millimicrons, and index of refraction being in the range of about 200 to 400, the uniformity of thickness of said platelets being such that at least about 80% of the total plate area does not dilfer in thickness by more than i% of the average platelet thickness so that colors are produced by optical interference phenomena upon reflection and transmission of light by said platelets.

3. The light-transmitting, macro-producing platelets as efined in claim 2 in which said platelets are composed of lead hydrogen .arsenate.

4. The light-transmitting crystalline nacre-producing platlets as defined in claim 2 in which said platelets are composed of basic lead carbonate.

5. A nacreous composition for producing color by lightinterference phenomena comprising a light-transmitting supporting me rum, said medium be ing, as a color-producing substance therein, ligl'it-transinitting crystalline, nacre-producing platelets selected from the group consisting of basic lead carbonate, bismuth oxychloride, lead hydrogen arsenate, lead hydrogen phosphate and mercurous formate having uniform thicknesses and an index of refraction of at least 1.70, the multiplication product of s ckness expressed in millimicrons, and index of refraction being in the range of from about 260 to 2000, the uniformity of thickness of said platelets being such that at least about 88% of the total plate area does not dili'er in thickness by more than l0% of the average platelet thickness so that colors are produced by optical interference phenomena upon reflection and transmission of light by said platelets.

6. A nacrcous composition for producing color by lightiuterference phenomena comprising a light-transmitting I medium, said medium having, as a color-pro uci substance therein light-transmitting crystalline more-producing platelets selected from the group consisting of basic lead carbonate, bismuth oxychloride, lead hydrogen arsenate, lead hydrogen phosphate and merurous formate, having uniform thicknesses, and an index of refraction of at least 1.70, the multiplication product of thickness expressed in millimicrons, and index of refraction bein in the range of about 260 to 460, the uniformity of thickness of said platelets being such that at least about of the total plate area does not difier in thickness by more than *;l0% of the average platelet thickness so that colors are produced by optical interference phenomena upon reflection and transmission of light by said platelets.

7. The nacreous composition as defined in claim 6, in which said platelets are composed of basic lead carbonate.

8. The composition as defined in claim 6, including a light-transmitting plastic supporting medium for said crystalline name-producing platelets, said medium having an index of refraction of from 1.4 to 1.65.

9. A colored nacreous article of manufacture comprising a light-transmitting supporting medium having as a color and nacre-producing substance therein light-transmitting crystalline nacre-producing platelets selected from the group consisting of basic lead carbonate, bismuth oxychloride, lead hydrogen arsenatc, lead hydrogen phosphate and mercurous forrnate, having uniform thicknesses and an index of refraction of at least 1.70, the multiplication product of thickness expressed in millimicrons, and index of refraction bein in the range of about 200 to 400, the uniformity of thickness of said platelets being such that at least about 86% of the total plate area does not differ in thickness by more than :10% of the average platelet thickness so that colors are produced by optical interference phenomena upon reflection and transmission of li ht by said platelets.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES American Ink Maker, vol. 23, No. 1, luly 1959, pages 5 and 6. 

5. A NACREOUS COMPOSITION FOR PRODUCING COLOR BY LIGHTINTERFERENCE PHENOMENA COMPRISING A LIGHT-TRANSMITTING SUPPORTING MEDIUM, SAID MEDIUM HAVING, AS A COLOR-PRODUCING SUBSTANCE THEREIN, LIGHT-TRANSMITTING CRYSTALLINE, NACRE-PRODUCING PLATELETS SELECTED FROM THE GROUP CONSISTING OF BASIC LEAD CARBONATE, BISMUTH OXYCHLORIDE, LEAD HYDROGEN ARSENATE, LEAD HYDROGEN PHOSPHATE AND MERCUROUS FORMATE HAVING UNIFORM THICKNESS AND AN INDEX OF REFRACTION OF AT LEAST 1.70, THE MULTIPLICATION PRODUCT OF THICKNESS EXPRESSED IN MILLIMICRONS, AND INDEX OF REFRACTION BEING IN THE RANGE OF FROM ABOUT 200 TO 2000, THE UNIFORMITY OF THICKNESS OF SAID PLATELETS BEING SUCH THAT AT LEAST ABOUT 80% OF THE TOTAL PLATE AREA DOES NOT DIFFER IN THICKNESS BY MORE THAN $10% OF THE AVERAGE PLATELET THICKNESS SO THAT COLORS ARE PRODUCED BY OPTICAL INTERFERENCE PHENOMENA UPON REFLECTION AND TRANSMISSION OF LIGHT BY SAID PLATELETS. 